In the manufacture of semiconductor integrated circuit (IC) devices, a technique known as lithography is used for transferring ultrasmall circuitry patterns onto a semiconductor wafer. The lithography technique typically includes a projection exposure apparatus, which loads a photomask and transfers the patterns located on the photomask onto a semiconductor wafer to expose portions of a photosensitive material located on the wafer. These patterns ultimately result in component structures, e.g., transistors, capacitors, diodes, microelectromechanical systems (MEMS) devices, heating elements, optoelectrical devices, etc. Since the exposure area of the photomask is smaller in size than the area of the wafer, a wafer surface is typically divided into a plurality of “shots” that are conducted in a stepping fashion across the wafer.
Depending on the type and size of structures that are to be fabricated, different types of photomasks can be used. For example, the photomask may be a binary mask, a gray scale mask, or a phase shift mask. Binary masks typically consist of a quartz substrate with a patterned chromium layer located over it. This mask has been used extensively over the years in fabricating semiconductor devices. A gray scale mask can be used where a slope or stepped configuration needs to be incorporated into a layer of a device component or structure. In such instances, the chromium layer can be patterned from progressively smaller to larger exposure areas, which effectively form the slope or stepped edge configuration in the targeted wafer substrate or layer during a plasma etch.
In recent years, however, device patterns have been made progressively smaller and denser to meet the demand for higher performance devices. To achieve the reduced feature sizes, manufacturers have had to turn to using much smaller and denser patterns on photomasks. Consequently, shorter wavelengths are needed to properly expose these patterns onto an underlying photosensitive material.
As an alternative to previous photomasks and to accommodate the shorter wavelengths, phase shift masks have been developed. The phase shift mask is a mask having a translucent film formed on a transparent plate for attenuating the exposing light and shifting the phase. Typically, the transmission of the exposure light through the film is desirable in the range between approximately 1% and 40%. The light transmitted through this film is adjusted to have certain phase differences from the light that does not pass through the film. The best phase difference for achieving the highest resolution is at about 180° and odd multiples thereof. However, the resolution can be improved when the phase is approximately 180°+/−0.90°. When using a phase shift mask, the resolution may be improved by approximately 5% to 20%, thereby providing manufacturers with the ability to manufacture smaller and denser device components.
These binary, gray scale, and phase shift masks have been found to work well in fabricating different types of devices.